Pulse duration modulation, often referred to as Class D modulation, is a scheme for increasing the efficiency of power amplifiers. Increasing the efficiency of power amplifiers is especially valuable in hearing aids, where the tiny cell must provide all the power for days, weeks, or months. An estimated 30-50 million hearing aids have incorporated the Class D hearing aid amplifier described by Killion in U.S. Pat. Nos. 4,592,087 and 4,689,819, which are incorporated by reference herein in their entirety, often more than doubling the battery life as a result. These Class D hearing aid amplifiers are generally characterized by a low-distortion push-pull balanced bridge output of pulse-duration-modulated pulses, and work extremely well with an inductive load that can store energy as described in the above patent disclosures and also described briefly in Killion 1991 [Audio, v75, No. 1, 42-44] and in further detail in Carlson 1988 [Hearing Instruments, v39, pp 30-32].
Killion, et al., in U.S. Pat. No. 5,099,856, which is incorporated by reference herein in its entirety, introduced a system for transmitting audio signals via light. The system includes a transmitter module, a fiber optic cable, and a receiver. The transmitter module converts an input signal into a Class D modulated series of light pulses. The fiber optic cable isolates the transmitter and receiver from common sources of electromagnetic interference. The receiver receives the light pulses and converts them into a replica of the original signal. The system has been implemented, for example, as an isolation amplifier for Auditory Brainstem Response measurements where interference is a constant problem. With this approach, however, the efficiency of a push-pull, balanced bridge output is lost. Instead, the light source, an LED, is driven directly with no intervening energy storage device.
A modulated light beam can also be used to transmit sufficient audio frequency current to a magnetic hearing aid receiver or “Earlens” vibrator on the eardrum of a wearer, as described in the U.S. Pat. No. 8,715,152 and U.S. Publication No. 2010/0048982 A1 by Puria et al, which are incorporated by reference herein in their entirety. In one example of such an application, a modulated stream of pulses may produce an average LED current of 10 mA, with peak current of 20 mA. Although a Class A driver works in this application, the LED light output versus current generally shows a non-linear characteristic, with less efficiency at lower currents, creating a distortion in the demodulated signal. For this reason, some form of pulse modulation, such as Class D pulse duration modulation, can provide greater linearity between the averaged light output and the input signal because each pulse—long or short—is at maximum light-output efficiency. If Class D peaks are set to 20 mA, then 100% ON time corresponds to a continuous current of 20 mA, 0% ON time produces zero current, and 50% ON time corresponds to an average current of 10 mA. This often is the zero-signal condition. In this way, the equivalent of a Class A waveform replication of the amplified input signal is obtained after the received light pulses sensed with a photodetector diode or transistor are averaged to produce an audio signal.
Another problem with Class A power amplifiers is a relatively large battery drain. Specifically, the idling current at zero signal is one half of the peak current, allowing a maximum sine wave waveform to go from the idling current to twice the idling current on one half of the peak waveform and from the idling current to zero current on the other half. Changing to pulse duration modulation does not improve the situation in the case of a resistive or LED load. In the above LED example, the same average current drain of about 10 mA is needed in either case. By using a voltage doubler, 10 mA at 2.2 V calls for approximately 20 mA from a 1.3 V Zinc Air cell, or a total of 26 mW. Ignoring other power consumption, this would correspond to only 23 hours life on the most powerful 675 Cochlear Zinc Air hearing aid cell. In other words, a new battery is needed at the start of each day with either Class A or Class D output.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.